linux-kernel - RE: [RFC][PATCH 0/4] ftrace: Use -mfentry when supported (this is for x86

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Date:	Fri, 18 Feb 2011 15:36:35 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Dominique Toupin <dominique.toupin@...csson.com>
Cc:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Frederic Weisbecker <fweisbec@...il.com>,
	"H. Peter Anvin" <hpa@...or.com>, Andi Kleen <andi@...stfloor.org>,
	"2nddept-manager@....hitachi.co.jp" 
	<2nddept-manager@....hitachi.co.jp>
Subject: RE: [RFC][PATCH 0/4] ftrace: Use -mfentry when supported (this is
 for x86_64 right now)

On Fri, 2011-02-18 at 15:10 -0500, Dominique Toupin wrote:
> My understanding is stop_machine will stop all processors for many ms.

s/ms/us/


> Even if most of our systems are not hard real-time they are soft real-time and stopping all cores for a few ms is not allowed.
> We can stop a few threads while we are jump patching but all processors is too much for us.

I think I could hit a single ms if we enable full function tracing which
disables ~22,000 functions in one shot. But if you enable full function
tracing, the kernel can slow down quite drastically, and that would even
be more problematic than a single ms hic-up. As hackbench showed a %150
slowdown when function tracer was running.

Now the last measurements I took was a few years ago and it was on a 4
CPU box. Perhaps stop_machine() may be a bit more expensive on a 1024
CPU box.

-- Steve


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